Copper-Catalyzed Annulation of Electrophilic Benzannulated Heterocycles with 2-Aminopyridine and 2-Aminoquinoline: Direct Access toward Polyring-Fused Imidazo[1,2-a]pyridines

Abstract

We have developed a direct method for the synthesis of polyring-fused imidazo[1,2-a]pyridines via a coppercatalyzed annulation of electrophilic benzannulated heterocycles with 2-aminopyridine and 2-aminoquinoline. From 3-nitroindoles and 2-aminopyridine, we could synthesize tetracenes, viz., indolefused imidazo[1,2-a]pyridines, and by starting from 2-aminoquinoline, we could generate pentacenes, viz., indolo-imidazo[1,2a]quinolines. In addition, we could also extend the methodology toward the synthesis of benzothieno-imidazo[1,2-a]pyridines starting from 3-nitrobenzothiophene. Furthermore, the basic photophysical properties of these synthesized heteroacenes were evaluated.

Annulation reactions of electrophilic benzannulated heterocycles towards heteroacenes

“…We also used copper catalyzed annulation reaction of 2-aminopyridine for the synthesis of benzothiophene-fused imidazo[1,2- a ]pyridine by using 3-nitrobenzothiophene as substrate (Scheme 32). 47 From this reaction, we could observe only a trace amount of the product.…”

“…The reaction of N -protected 3-nitroindoles with 2-aminopyridines in the presence of CuI afforded indole-fused imidazo[1,2- a ]pyridines. 47…”

“…The copper catalyzed annulation protocol could also be extended to obtain pentacenes. 47 Electrophilic indoles on treatment with 2-aminoquinoline in the presence of CuI in DMF proceeded to afford indolo-imidazo[1,2- a ]quinolines (Scheme 21). Finally, we also evaluated basic photophysical properties of these heteroacenes to assess their suitability in material applications.…”

Tandem Reactions of Electrophilic Indoles toward Indolizines and Their Subsequent Transformations through Pd(II)-Mediated C–H Functionalization to Access Polyring-Fused N-Heterocycles

“… 10 3-Nitroindoles are prone to react with 1,3-dipoles in dipolar cycloaddition reactions toward annulated heterocycles. 11 Due to our continued fascination with the reactivity of electrophilic benzannulated heterocycles, 12 we were able to synthesize functionalized pyrrolo[2,1- a ]isoquinolines by treating electrophilic indoles with isoquinolinium methylides through a domino dipolar cycloaddition-ring opening reaction 13 ( Scheme 1 e). Inspired by the above reports, we envisioned that novel indolizine derivatives could be accessed from the reactions of electrophilic indoles and pyridinium N -ylides ( Scheme 1 f), which presents an alternative strategy for synthesizing complex derivatives of indolizines.…”

HPW-Catalyzed environmentally benign approach to imidazo[1,2-a]pyridines

“…Some recent synthetic approaches to imidazo[1,2- a ]pyridine scaffolds include synthetic pathways of transition metal-catalyzed reactions [ 14 ], cyclization [ 15 ], condensation [ 16 ], heteroannular [ 17 ], and photocatalytic reactions [ 18 ]. These approaches usually involve non-trivial reaction conditions and the employment of relatively complex starting materials [ 19 ].…”